1. Field of the Invention
The present invention relates to segmented reflector antennas for producing overlapping antenna beams from separate feeds without incurring cross-coupling between feeds and power loss. More particularly, the reflecting surface is segmented to provide separate images of the far field in the vicinity of the original focal surface of the antenna. Feeds disposed at corresponding locations on each of the far field images produced by each of the segments provide separate beam footprints which overlap each other by predetermined amounts.
2. Description of the Prior Art
Reflector antennas can produce a multiplicity of beams in different directions by feeding the reflectors with different horns placed at different locations. However, the resultant beams do not, in general, overlap to provide approximately uniform coverage over the field of view of the antenna. Conventional methods, employed to make the coverage more uniform, involve making the feed horns very small in order to pack them closer together resulting in a power loss due to reflector spillover and mutual coupling between feeds. By sharing feed horns between two or more beams the spillover can be reduced, but the mutual coupling remains while waveguide feed networks are made more complicated.
A typical prior art multiple bean antenna is disclosed in U.S. Pat. No. 3,914,768 issued to E. A. Ohm on Oct. 21, 1975. There, a multiple beam antenna configuration is disclosed which supports a plurality of angularly displaced but well-isolated beams and exhibits essentially zero aperture blockage. A plurality of feed horns are clustered around the on-axis focal point of an offset Cassegrainian antenna in which the subreflector is displaced from the aperture to avoid blockage. This hyperbolic subreflector is sized and shaped to accommodate the plurality of beams and the feeds are individually aimed toward the subreflector so that all of the beam centers impinge upon the common effective center of the main parabolic reflector.
Another prior art multiple beam antenna is disclosed in U.S. Pat. No. 4,236,161 issued to E. A. Ohm on Nov. 25, 1980. There a multiple beam antenna arrangement is disclosed which provides a plurality of communication beams for illuminating a predetermined zone. Plural feed horns are disposed on the focal surface of an offset antenna, which horns are energized in cluster groups which produce contiguous beams of predetermined frequencies and polarizations. Adjacent cluster groups operate at diverse frequencies and share at least one feed horn to provide area coverage of the zone. Orthogonally polarized spot beams cover high traffic areas such as cities.
Another prior art multiple beam scanning antenna is disclosed in U.S. Pat. No. 4,315,262 issued to A. Acampora et al on Feb. 9, 1982. There in FIGS. 6-9 an array antenna is shown for limited scanning over multiple independent linear strip subdivisions of a total service area. More particularly, each row of feed elements of the feed array acts essentially as a line source which radiates a wavefront that is transformed by a reflector into a spot beam in the far field. This spot beam can then be scanned over a linear portion of the far field.
The problem remaining in the prior art is to provide a multibeam antenna arrangement wherein beams can be made to overlap each other to provide approximately uniform coverage of the field of view of the antenna while avoiding mutual coupling between feeds.